1. Field of the Invention
The present invention relates to a motor drive apparatus, and particularly to a motor drive apparatus driving a 3-phase brushless motor without using a Hall sensor.
2. Description of the Background Art
Conventionally, a motor drive apparatus which drives a 3-phase brushless motor without using a Hall sensor is known. In this motor drive apparatus, as the rotor rotates, its position is detected using not a Hall sensor but an induced voltage generated in a coil of a stator by the rotation of the rotor. Based on the detection result, 3-phase PWM voltages 120° out of phase with each other are applied to 3-phase coils (for example, see Japanese Patent Laying-Open No. 2003-047280).
More specifically, in such a motor drive apparatus, the current supply to a coil of a position detection phase is timed, as prescribed, to be interrupted, and a zero cross point of a coil terminal voltage of the position detection phase is detected after a lapse of a certain mask period. Based on the detection result, the 3-phase PWM voltages are then applied to the 3-phase coils. Although the coil current should be 0 A in order to detect the zero cross point of the coil terminal voltage, the coil current cannot immediately become 0 A even if the current supply to the coil is interrupted. Therefore, the certain mask period must be introduced.
On the other hand, when the 3-phase brushless motor is driven quietly without decreasing its rotation efficiency, an electrical angle of each phase when current conducts should be brought from 120° close to 180°. This causes the possible length of the mask period to be shortened. In addition, an increase in the coil constant of the motor causes the attenuation of the coil current to be slowed down. Therefore, the coil current cannot become 0 A by the time of the detection of the zero cross point. In this case, the zero cross point cannot be accurately detected, so that it becomes difficult to control the motor.